For further information contact Nancy R. Pataky, Extension Specialist and Directorof the Plant Clinic, Department of Crop Sciences, University of Illinois at Urbana-Champaign.

University of Illinois Extension provides equal opportunities in programs and employment.

Figure 1. Alternaria leaf spot or blight (R.K.

Jones, NC State photo).

Figure 2. Alternaria flower blight (J.L. Forsberg

photo).

COMMON LEAF DISEASES OF ZINNIA

Zinnias (Zinnia elegans) are one of the most popular summer- and autumn-flowering plants. Threecommon leaf spot diseases mar the beauty of this bedding plant. All three diseases are favored by warm,wet or very humid weather, overhead sprinkling, excess shade, and crowding of plants.

ALTERNARIA LEAF SPOT OR BLIGHT

This widespread and disfiguring disease is caused by the fungusAlternaria zinniae. The causal fungus also infects other plantssuch as China aster, lettuce, sunflower, tobacco, and tomato. TheAlternaria fungus overwinters on seed and in soil.

Small, round, reddish brown spots with white to grayish whitecenters form on the upper leaf surface. Later, the lesions becomefairly large (up to 10 mm in diameter), irregular, dark reddishbrown or purple, and dry (Figure 1). The center of an older spotmay drop out leaving a ragged hole. Heavily infected leaves turnbrown and dry. The dead tissue has a tendency to crack and tearduring wind and rain storms. Similar spots form on the petiolesand on the stems at or between the nodes. The lesions may girdlethe stem, often at a node, causing the upper portion of the plant towilt and die back. Dark brown to black cankers with sunkencenters are common near the soil line. Affected plants often wilteven when the basal cankers do not completely encircle the stem.

Infected roots mayturn dark gray,rot, and slough offr e s u l t i n g i nwilting and deathof the plant. Seedlings may also wilt and collapse (damp-off). Small (1 to 2 mm) but enlarging brown spots,sometimes with grayish white centers, appear on the petalsof the ray flowers. The lesions may enlarge and involveentire petals, causing an unsightly blighting of the flowers(Figure 2).

DEPARTMENT OF CROP SCIENCESUNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

report on

PLANT DISEASE

RPD No. 655October 1987

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Figure 3. Cercospora lea f spot (Florida

Dept. of Agriculture, Division of Plant

Industry photo).

Figure 4. Bacterial leaf spot showing large

areas of decaying tissue (Florida Dept. of

Agriculture, Division of Plant Industry photo).

CERCOSPORA LEAF SPOT

This disease, which very closely resembles Alternaria leaf spot,is caused by the fungus Cercospora zinniae. It sometimes occurstogether with Alternaria zinniae on the same leaf.

Fairly large, almost round, reddish brown or dark purple spots,with a light gray or even white center, form in the leaves (Figure3). As with Alternaria leaf spot, the center of the lesion may fallout leaving a hole. Severely infected leaves turn brown and dry.The dead tissue may crack and tear.

BACTERIAL LEAF SPOT

This disease is caused by the bacterium Xanthomonas campestrissub. Sp. zinniae. It first appears as small (1 to 2 mm), diffuse,translucent spots surrounded by broad yellowish halos. Underwet conditions the lesions slowly enlarge to about 5 mm indiameter. The spots become angular to irregularly circular anddevelop a reddish center. The lesions may merge to formirregular dead areas, 0.5 to 1.0 cm long, that may crack as theydry (Figure 4). During very humid weather, small brown spotsmay form on the ray flowers. If severe, the flower heads areseriously disfigured and may completely decay.

Control

1. Purchase only disease-free, healthy transplants or start withfresh, disease-free seed. If unsure about the seed, it can befreed of Alternaria and Cercospora by soaking in hot water(125°F) for 30 minutes before planting. Immediately aftertreatment, plunge the seed into cold water and drycompletely before planting. Seed over a year old may beseverely injured by this treatment. Seed suspected of beinginfected with the bacterial leaf spot organism may be soakedfor 2 minutes in a household bleach solution (0.52 percentsodium hypochlorite; 10 percent Clorox) just before planting. After the hot water or householdbleach soak, treat the seed with a captan seed protectant fungicide. The captan will aid in preventingdamping-off.

2. Plant in full sun or semi-shade in a deep, rich, fertile soil that has been well prepared. Fertilize beforeplanting based on a soil test.

3. Space plants for good air circulation based on maturity. Giant forms of zinnias, which grow up to3 feet high, should be planted 1 ½ to 2 feet apart; the free-flowering, smaller-flowered Lilliputianvarieties, which grow up to 18 inches tall, may be planted 1 foot apart.

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4. Thoroughly water plants in hot dry weather being careful to keep the water off the foliage. Anorganic mulch around the plants will help to retain moisture and keep the soil from packing duringheavy rains or watering.

5. Practice thorough sanitation. Collect all crop debris after the growing season is over. Burn orcompost this plant refuse.

6. Grow zinnias in the same area only once in 3 years.

7. Control Alternaria and Cercospora leaf spots by thoroughly spraying seedlings and young plants witha protective fungicide at about 7- to 14-day intervals during warm, wet, or very humid weather. Besure to cover the underleaf surfaces with each spray. Applications should begin just before or whenthe first disease symptoms appear. Fungicide recommendations for zinnias are given in IllinoisCommercial Landscape and Turfgrass Pest Management Handbook or Illinois Home, Yard, andGarden Pest Guide.